Spring brake system with a constant force single stroke pumping means

ABSTRACT

A power brake system for a parking brake having a hydraulically operated spring brake with a manual control. A single stroke pump responsive to a pressure differential created by the flow of a first and second fluid under pressure regulated by the manual control is capable of pressurizing hydraulic fluid. This pressurized hydraulic fluid is adapted to overcome the resiliency of the spring brakes to maintain the parking brake in a released position. A flow control device adjacent the manual control will assure that a constant differential pressure is maintained in the pump to stabilize the force of the pressurized hydraulic fluid. An indicator responsive to the pressure of the hydraulic fluid provides a visual indication of the operational mode of the spring brake to an operator.

United States Patent 1 1 Gardner et a1.

[75] Inventors: Delbert J. Gardner; Louis P.

Rossigno; Leo H. McCormick, .lr., all of South Bend, Ind.

[73] Assignee: The Bendix Corporation, South Bend, Ind.

[22] Filed: Aug. 2, 1971 [21] Appl. No.: 168,226

[52] U.S. Cl ..60/53.4, 188/170. 137/6275.

[51] Int. Cl. F151: 7/00 [58] Field of Search 60/545, 54.6;

[56] References Cited UNITED STATES PATENTS 3,646,758 3/1972 Cripe60/54.6 P

3,617,096 11/1971 Grabb 303/9 3,390,921 7/1968 Klimek 303/56 3,201,1768/1965 l-lager 60/545 P FOREIGN PATENTS OR APPLICATIONS 765,079 l/1957Great Britain 60/54.6 P

INTAKE MAN/FOLD pm mun [451 July 24,1973

573,144 3/1959 Canada 60/595 P Primary Examiner-Martin P. SchwadronAssistant Examiner-A. M. Zupcic Att0rney--Leo H. McCormick, Jr.

draulic fluid is adapted to overcome the resiliency of the spring brakesto maintain the parking brake in a released position. A flow controldevice adjacent the manual control willassure that a constantdifferential pressure is maintained in the pump to stabilize the forceof the pressurized hydraulic fluid. An indicator responsive to thepressure of the hydraulic fluid pro- 1 vides a visual indication of theoperational mode of the spring brake to an operator.

3 Claims, 2 Drawing Figures SPRING BRAKE SYSTEM WITH A CONSTANT FORCESINGLE STROKE PUMPING MEANS BACKGROUND OF THE INVENTION Spring brakeshave been proposed as one part of the dual braking systems inanticipation of the Federal Highways Safety Laws. Initially, the springbrake was operated by an inversion valve connected to the same source ofpressurized fluid used to operate the entire braking system. However, ifthis pressurized fluid were lost, the spring brakes would automaticallybe applied, as disclosed in US. Application No. 797,530, filed Feb. 7,I969, now US. Pat. No. 3,599,761, and incorporated by reference. Lateras disclosed in US. Application No. 28,483, filed Apr. 15, I970, nowU.S. Pat. 3,617,097, and incorporated by reference, a split full powerbraking system having a pump-driven off the crankshaft was developed toindependently supply the inversion valve with a pressurized fluid tooperate .the spring brakes. Still later as disclosed in US. ApplicationNo. 91,641 filed Nov. 23, 1970, now US. Pat. No. 3,703,077, and US.Application No. 126,020 filed Mar. 19, 1971, vacuum powered pumps weredeveloped to conserve and reduce the number of potential devicesoperated by the power produced by crankshaft.

In these vacuum pumping devices it is necessary to have storage vesselswith a flow controlled inlet port to maintain the fluid force requiredto release the spring barke since these pumps modulated eitherautomatically or manually until a predetermined hydraulic pressure forthe system was achieved.

SUMMARY OF THE INVENTION In internal combustion engines, a partialvacuum will normally be produced at the intake manifold. The intensityof this partial vacuum will be the greatest when the accelerator pedalis released, as during periods of braking and while the vehicle isstopped, and the least during periods of continual acceleration or pullup an incline. The intensity of the partial vacuum is directly relatedto the acceleration pedal whose position con trols the intake air intothe carburetor while the displacement of air by the pistons remainsconstant.

To best utilize the partial vacuum produced at the intake manifold inconjunction with a spring brake means used as a parking brake, we havedevised a braking system with a manually controlled single stroke pumpmeans that supplies a constant output force. The pump means consists ofa housing having an internal cavity divided by wall means into a frontchamber and a rear chamber. Piston means attached to the wall means isretained in a bore of a cylinder containing hydraulic fluid. Thecylinder is connected to the spring brake means. A manual control has afirst inlet port connected to air under atmospheric pressure, a secondinlet port connected through flow control means to the intake manifoldproducing a partial vacuum and an outlet port connected to the frontchamber. The rear chamber having air under atmospheric pressure beingfreely admitted through a filter at all times.

During a first mode of operation when it is desired to have the springbrakes applied, the first inlet port of the manual control is openedallowing air at atmospheric pressure to flow into the front chamber.With air at atmospheric pressure in both the front chamber and the rearchamber, a retainer spring moves the wall means and the connected pistonmeans from a pressurizing position in the bore of the cylinder tothereby permit a resilient member to actuate the spring brakes. Thespring brakes are normally used as a parking brake, but in the event offailure in service, the brakes could also be used as an emergency brake.

When an operator desires to release the spring brakes, the manualcontrol is switched to the second mode of operation. In the second modeof operation the second inlet port is opened and the first inlet portclosed. As the vehicle produces a partial vacuum at the manifold, theair in the front chamber is evacuated to create a pressure differentialacross the wall means. When the pressure differential reaches apredetermined value, the piston means will move in the bore of thecylinder and pressurize hydraulic fluid sufficiently to overcome theresilient member and release the spring brakes.

Once the maximum partial vacuum capable of being produced has evacuatedthe front chamber to create the pressure differential, flow controlmeans in the partial vacuum supply conduit will prevent changes in thispressure differential caused by changes in the intensity of the partialvacuum produced. Thus, the pressurizing force exerted by the pistonmeans will remain relatively constant.

Indicator means connected to receive a signal from a pressure responsivedevice will give an operator a vi-. sual indication of the hydraulicpressure in the system. In the first mode of operation a continualsignal will be energized and in the second mode of operation the signalwill be completely de-energized. As the hydraulic pressure in the systemapproaches the first mode of operation, an intermittent signal willalert the operator that the spring brakes are about to be activated.

It is therefore an object of this invention to provide an emergency andparking brake system with the means for releasing and applying springbrake means without graduation.

It is another object of this invention to provide a braking systemhaving pumping means manually controllable to provide a' constantpressurizing force from a single stroke.

It is still a further object to provide a braking system having a singlestroke pumping means with means to give an operator an indication of thepressure of the operational fluid supplied spring brake means.

These and other objects will be readily recognized by those who readthis specification and view the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows an auxiliary brakingsystem for operating a parking spring brake means with an enlargedsectional view of the control and pumping means used to provide theoperational power.

FIG. 2 shows a secondary embodiment of the braking system with controlmeans for vacuum suspending the pumping means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, a wheelbrake assembly 10 is shown as receiving a first source of hydraulicfluid through conduit 12 for operating a service brake l4 and a secondsource of hydraulic fluid through conduit 16 for operating spring brakemeans 18 as described in copending US. Application No. 797,530. Thepower for operating the spring brake means 18 is derived from a vacuumoperated pumping means which is regulated by manual control means 22.The manual control means 22 is connected by a conduit 24 to the intakemanifold 26 of a vehicle. When the manual control means 22 is in a firstposition (as shown in the drawing), a first mode of operation will existwhere pumping means 20 will be deenergized. With pumping means 20de-energized, the second source of hydraulic fluid will be unpressurizedto thereby permit a resilient member (not shown) to activate the springbrake means 18. With the spring brakes activated, rotary motion betweendisc 28 and spring brake means 18 will be reduced because of thefrictional drag between pad 30 and the disc 28.

After the vehicle has stopped and it is desired to move the vehicle, themanual control will be moved to a second position to commence a secondmode of operation where the pumping means 20 is energized through vacuumproduced at the manifold 26. Through energization of pumping means 20,the second source of hydraulic fluid will be pressurized andcommunicated through conduit 16 to the spring brake means 18.

A pressure switch 32 of a type fully described in copending U.S.Application Ser. No. 784,775, filed Dec. 18, 1968, now U.S. Pat. No.3,593,265, and incorporated herein by reference, is located in conduit16 between pumping means 20 and spring brake means 18. Switch 32 has aninternal movable shaft (not shown) which is responsive to the hydraulicfluid pressure in the conduit 16. When the conditions for the first modeof operation are present in the braking system and the hydraulic fluidin conduit 16 unpressurized, the shaft will be moved by a spring toclose an electrical contact switch which will cause an indicator device34, either a warning light (as shown) which continually glows or flashesor an audible signal (not shown) to be energized for alerting anoperator of this pressure condition. With the indicator 34 energized, anoperator will be assured that the spring brakes will soon be activated.When the conditions for the second mode of operation are present, thehydraulic fluid pressure in conduit 16 will overcome the spring holdingthe shaft against the contact switch to thereby de-energi'ze theindicator 34. When indicator 34 is de-energized, the operator will knowthe spring brakes have been released and the vehicle can be movedwithout damaging the wheel assembly 10.

In more particular detail, the vacuum operated pumping means 20 consistsofa power transmitting section 36 an a hydraulic fluid intensifyingsection 38 connected to the spring brake means 18 by conduit 16.

The power transmitting section 36 has a housing 40 with an internalcavity. A wall diaphragm means 42 has a first bead 44 held by an annularrib 46 in the housing 40 and a second bead 48 held to a stiff internaltwopiece plate 50 which is clamped together to divide the cavity into afront chamber 52 and a rear chamber 54. The front chamber 52 isconnected to the manual control means 22 by conduit 53 and the rearchamber 54 is connected to the atmosphere through filter 55. A flexibleportion 57 located between the first and second beads 44 and 48 willpermit the wall or diaphragm means 42 to be freely moved axially withinthe cavity. A resilient member 56 is concentrically located on plate 50to surround a push rod 58 attached to plate 50. A pair of nuts 60 and 62are threaded on the push rod 58 in such a manner as to hold the twopiece plate 50 together. The push rod 58 extends through the housing 40into the intensifying section 38 to transmit any force created bymovement of the wall or diaphragm means 42 caused by a pressuredifferential between the front chamber 52 and the rear chamber 54,thereinto.

The fluid intensifying section 38 has a cylindrical body 64 attached tothe housing 40 by bolts 66 (only one is shown). The cylindrical body 64has an internal bore 68 located in an axially extending chamber 70 fromwhich hydraulic fluid is forced through outlet port 72 by displacementpiston means 74 attached to push rod 58. To replace the loss ofhydraulic fluid in chamber 70 upon piston means 74 moving to the right,as shown in the drawing, to transmit pressurized hydraulic fluid to thespring brake means 18, a compensating port 76 is connected to inlet port78 in communication with reservoir 80. The change in volume of fluid inchamber 70 could occur from loss of hydraulic fluid in the system.During power activation, the compensation port 76 is closed by a poppetmember 82, retained in an enlarged section 84 of the piston 86, seatingon shoulder 88. The push rod 58 to which poppet member 82 is attached,projects into the enlarged section 84 and is loosely fastened to thepiston 86 by cross pin 90. The cross pin 90 is fixed to the push rod 58but moves in a slotted section 92 on the piston 86. The length of theslotted section 92 is designed to permit the poppet member 82 to beunseated when the pin engages the rear end of the slot and seated onshoulder 88 at the front end of the slot. The cross pin 90 is preventedfrom coming loose by a snap ring 94 positioned in a groove on the outersurface of the piston 86 overlying the ends of cross pin 90. A guidebearing 96, retained by a snap ring 98, maintains push rod 58 inalignment with bore 68 in the intensifying section 38. Seals 93 and 95surrounding bearing means 96 prevent hydraulic fluid from entering intothe front chamber 52.

The manual control means 22 consists of a housing 100, which will beadapted to be mounted on the dashboard 103 of a vehicle (not shown),with an internal chamber 102 having a first inlet port 104 connected tothe atmosphere through filter 106, a second inlet port 108 connected toa source of vacuum through conduit 24, such as the intake manifold 26 ofan internal combustion engine and an outlet port 112 connected byconduit 53 to the front chamber 52 of the pumping means 20. A partition116 separates the outlet port 112 from the second inlet port 108. Thepartition 116 has a central opening 120 surrounded by a seat 122. Arubber disc 124 is held in a retainer 126 and is urged against seat 122by resilient means 128. A shaft 130, located in bore 132 connected tothe internal chamber 102, has a concentric central bore 134 whichextends from end 136 positioned in opening 120 of partition 116 to apoint where a passage 138 is adjacently connected to the first inletport. A resilient member 140 is held between the partition 116 and ashoulder 142 on shaft 130. The resilient member 140 urges shaft 130against an eccentric face 142 and head 144 pivotally retained on pin146. A shaft 148 is connected to head 144 with a knob 150 to enable anoperator to move shaft 130 between a first and second operatingposition.

Check valve means 152 located between the intake manifold 26 and thesecond inlet port 108 has a flow chamber 154 with an inlet 156 and anoutlet 158. A seal surrounds the outlet 158 and a face member 162 isheld against seat 160 by resilient member 164. Flow through the checkvalve means 152 is limited to a single direction, permitting only thepartial vacuum created at the manifold to evacuate the front chamber 52when the manual control means 22 is in the second mode of operation.Thus, once the front chamber 52 has been evacuated by the maximumpartial vacuum capable of being produced at the manifold 26, furtherflow through the check valve means 152 is restricted. The pressuredifferential across the wall means 42 will now be uneffected by changesin the level of the partial vacuum produced and a constant output forcewill suecessively be re-produced for providing the power to hold the padof the spring brake 18 out of contact with disc 28.

MODE OF OPERATION or THE PREFERRED EMBODIMENT When an operator turns onthe ignition switch 166 of the vehicle, if, after the vehicle has beenstopped for a period of time, the hydraulic pressure in the supplyconduit 16 going to the spring brake means 18 is below a predeterminedvalue, the indicator 34 will give a signal to alert the operator of thiscondition.

If the loss of hydraulic pressure has occurred because of an accidentalloss of vacuum from the front chamber 52 as when the manual control forthe spring brake means was not placed in the first mode of operation itwill be necessary for the partial vacuum produced at the manifold toestablish the operational pressure differential. In this position manualcontrol means 22 will hold shaft 130 against face 123 of the seal forthe second inlet port 108 by eccentric face 142 being positioned by knob150 as shown by the dashed lines at B. Once the intake manifold beginsto produce sufficient partial vacuum the front chamber 52 will beevacuated and the operational pressure differential established acrosswall means 42. This pressure differential will move piston means 86 inbore 68 to pressurize the hydraulic fluid therein which will overcomethe resilient bias 'of the shaft (not shown) of the switch 32. Switch 32will,'in turn, terminate the signal emitted from indicator 34. When thesignal from indicator 34 stops, the operator will know it is safe tomove his vehicle without partial vacuum at the manifold 26, the operatorwill movethe manual control knob 150 fromposition A to position B. Asthe control knob 150 moves from the first mode of operation at positionA to the second mode of operation at position B, head 144 pivots on pin7 146 and causes the eccentric face 142 to slide shaft 130 intoengagement with face 124 of the seal 123. With the tubular end of shaft136 held against face 124, passage of air at atmospheric pressurethrough the first inlet 104 is stopped and face 124 lifted off seat 122to open first inlet port 108. The partial vacuum produced at themanifold communicated through check valve means 152 can now evacuate theair from the front chamber 52. With partial vacuum in the front chamber52 and air at atmospheric pressure in the rear chamber 54, a pressuredifferential will be produced across wall means 42. This pressuredifferential will move the piston means 86 attached to wall means 42 topressurize the hydraulic fluid in chamber 70. When the piston the pads30 of the spring brake means 18 engaging disc means 86 reaches thebottom of its stroke sufficient hydraulic pressure will be produced torelease the spring brake means 18. Check valve means 152 will hold themaximum partial pressure produced at the manifold 26 in the frontchamber 52 to create a relatively constant pressure differential acrosswall means 42. The flexible portion 57 will permit the piston means 74attached to the wall means 42 to move in bore 68 with changes intemperature which effects the space volume of the hydraulic fluid. Thiscorresponding movement between temperature changes and piston means 74will maintain the pressurizing force holding the spring brake means 18released within predetermined limits at all times.

When an operator wishes to apply the spring brake means, the knob 150 ofthe manual control means 22 is shifted to position A producing the firstmode of operation. In this mode of operation, resilient means 140 willurge shaft away from face 124 permitting seal 123 to be placed on seat122 by resilient means 128. In this position air at atmospheric pressureenters through the first inlet port 104 into passage 138 and bore 134past the tubular end of shaft 130 and out the outlet port 109. Sinceoutlet 112 is connected by conduit 53 to the front chamber 52, wallmeans 42 is now suspended in atmospheric pressure permitting resilientmeans 56 to move piston means 86 from a pressurizing position. With thehydraulic fluid unpressurized, the spring brakes will be applied and themovement of the vehicle stopped. I

In the event that the engine has stalled and it would be desirable tomove the vehicle, a storage reservoir 168 can be placed in conduit 24between the check valve means 152 and the second inlet port 108. Thevacuum stored in this reservoir would permit several applications of thespring brake means to allow movement of the vehicle from one point toanother after the engine has stopped without engagement of the springbrake means 18.

It will be understood that in the embodiment shown in FIG. 1, the outlet112 of the manual control means 22 could be connected to the rearchamber 54 of the pumping means 20 and the front chamber 52 connected toair under atmospheric pressure as long as the second inlet 108 wasconnected to an air compressor (not shown). The air compressor beingcapable of raising the pressure of the first source of fluid aboveatmospheric pressure and the operation of the pumping means 20 would bethe same since a pressure differentialcould be created across wall means42 to move the piston means 86. By providing such a system withreservoir the air compressor could be operated on demand when theindicator 34 signaled the hydraulic fluid pressure was below apredetermined value.

In FIG. 2, elements that are the same as in FIG. 1 are designated withthe same numeral and further description is not deemed necessary.

In the embodiment shown in FIG. 2, control means 200 alternatelysupplies air at atmospheric pressure and partial vacuum produced at themanifold 26 through conduit 202 to the rear chamber 54 of the vacuumoperated pumping means 20 while the front chamber 52 is continually incommunication through conduit 204 with the partial vacuum in reservoir168. In a first mode of operation the manual control 206 of the controlmeans 200 is positionedto permit partial vacuum in the rear chamber tovacuum suspend the wall means 52 and allow resilient means 56 to movepiston means 74 releasing the pressurizing force on the spring brakemeans 10. With the pressurizing force removed, pads 30 will engage disc28 to frictionally stop the vehicle. In a second mode of operation themanual control 206 permits air at atmospheric pressure into the rearchamber creating a pressure differential across the wall means 42causing the piston means 74 to move and pressurize the hydraulic fluidto release the spring brake means 10.

In more particular detail, control means 200 consists of a housing 208having an atmospheric chamber 218, a control chamber 220 and a partialvacuum chamber 222. A first inlet port 210 connects the partial vacuumchamber 222 with reservoir 168 through conduit 212. A second inlet port214 freely permits air to the atmospheric chamber 218. An outlet port216 connects the control means 200 to conduit 202 going to the vacuumoperated pumping means 20.

A diaphragm 224 having an outer periphery 226 secured to the housing 208has an inner periphery 228 held between a first diaphragm plate 230 anda second diaphragm plate 232 on tubular shaft 234. A spring 236surrounds the first inlet port 210 and is held in recessed portion 238of the tubular shaft 234. The spring 236, through the tubular shaft 234,urges the second diaphragm plate against a diaphragm restriction plate240 which separates the partial vacuum chamber 222 from the controlchamber 220 in addition to limiting the movement of the diaphragm 224.

A tubular valve stem 242 extends from the control chamber 220 throughsealing surfaces 244 to the atmospheric chamber 218. A flange 246 on theend of the stem 242 in the control chamber provides a surface forholding spring 248. A cross pin 250 is adjustably secured by threads toa portion of stem 242 in the atmospheric chamber 218. The spring 248urges cross pin 250 against a cam surface 252 on the manual control 206.A closure member 254 is resiliently positioned against annular seat 256by spring 258. As shown in FIG. 2, the manual control 206 is positionedin the first mode of operation where partial vacuum from reservoir 168is communicated to the partial vacuum chamber 222 through tubular shaftand into the control chamber 220. Since the control chamber is incommunication with the rear chamber 54 of vacuum controlled pumpingmeans 20 through outlet port 216, the wall means 42 will be suspended ina partial vacuum. With wall means 42 suspended in a partial vacuum, thespring brake means 10 will now be applied in the same manner as recitedabove in the mode of operation of FIG. 1.

The partial vacuum chamber 222 is connected to a Bourdon tube gage 266located in chamber 262 by passage 264. An indicator 260 attached to theBourdon tube, of a known type or with temperature compensating means asshown' in US. Application No. 56,561, filed July 20, I970 andincorporated by reference, will move with respect to dial 268 to givethe operator a visual, indication of the pressure differential producingcapability across the wall means 42 as sensed by the level of partialvacuum produced at the manifold 26 and retained in reservoir 168.

When the manual control 206 is moved to the second mode of operation,cross pin 250 will slide on the cam surface 252 and move in slot 270under the influence of spring 248. When the manual control 206 has beensufficiently moved, the closure member 254 will seat on the end oftubular shaft 234 closing the communication between the partial vacuumchamber 222 and the control chamber 220. Upon further movement of themanual control 206, the closure member 254 will move off annular seat256 permitting air at atmospheric pressure to flow from the atmosphericchamber 218 to the control chamber 220. From the control chamber 220 theair at atmospheric pressure flows through the outlet port 216 by conduit202 to the rear chamber 54 to create a pressure differential across thewall means 42. The pressure differential will move the piston means 74to pressurize the hydraulic fluid and release the spring brake means 10in the same manner as described above with reference to FIG. 1.

Through our braking systems we have provided a spring brake means with asimple operational power source with manual control thereby reducingcost and the number of parts to effectively operate a demand emergencyand parking brake system.

We claim:

1. In a braking system for a vehicle having a hydraulically operatedspring parking brake, manually controllable pumping means forcontinually supplying fluid at a constant pressure to release saidspring parking brake in a first mode of operation and for eliminatingsaid constant pressure to activate said spring brakes in a second modeof operation, said manually controllable pumping means comprising:

a first housing having an internal cavity therein;

wall means including a flexible portion for dividing said cavity into afront chamber and a rear chamber, said front chamber being connected toa first source of fluid under pressure, said rear chamber beingconnected to a second source of fluid under pressure higher than saidfirst pressure, said flexible portion being adapted to move in responseto expansion of said fluid with changes in temperature in said firstmode to maintain the constant hydraulic fluid pressure on the springbrake within a predetermined range;

a cylinder having an internal bore with an inlet port and an outletport, said inlet port being connected to a reservoir containinghydraulic fluid, said outlet port being connected by a conduit to saidspring brake means;

piston means connected to said wall means and extending into theinternal bore of said cylinder;

poppet means attached to said piston means for preventing communicationbetween said reservoir and said bore during said first mode of operationand for allowing communication therebetween during said second mode ofoperation;

resilient means located in said front chamber for urging said wall meanstoward said rear chamber during said second mode of operation;

a second housing having a first chamber connected to said front chamber,a second chamber con nected to said rear chamber and a control chamberinterconnected to said first and second chambers;

valve means located in said control chamber having a tubular shaftsurrounded by a diaphragm for separating the second chamber fron thecontrol chamber and a tubular valve stem extending through the secondhousing from the first chamber into the control chamber, said tubularhousing having a resiliently positioned closure member on the endthereof adjacent said tubular shaft;

sensing means located in said second housing and connected to saidsecond chamber for visually indicating a differential pressure capableof being produced across said wall means in said first mode ofoperation;

flow control means connected to said second chamber of the secondhousing and the front chamber of the first housing for maintaining saidfirst source of fluid under pressure communicated thereto at a constantpressure during both the first and second modes of operation;

means responsive to the hydraulic fluid pressure being supplied to saidspring brake means during said first mode of operation for transmittinga first signal to an indicator device to inform the operator that thespring brakes are released and for transmitting a second signal duringsaid second mode of operation to inform the operator that the springbrakes are applied and need to be released before movement of thevehicle; and

manual control means connected to said tubular valve stem forselectively engaging said closure member with said tubular shaft inresponse to an operator for sequentially communicating said first andsecond chambers with 'said control chamber to develop a correspondingpressure differential across said wall means to move said piston meansin the internal bore of the cylinder and sufficiently pressurize thehydraulic fluid therein by a single stroke to establish said first modeof operation.

2. The power braking system, as recited in claim 1, wherein said firstsource of fluid under pressure is a partial vacuum and said secondsource of fluid is air at atmospheric pressure, said partial vacuumbeing connected to said first inlet port and said front chamber, saidair at atmospheric pressure being connected to said second inlet port,said valve means in the first operational mode permitting partial vacuumto be communicated from the first inlet port past the outlet port andinto the rear chamber to eliminate the pressure differential across thewall means and release the pressurizing force on the hydraulic fluid toallow the spring brakes to be applied. I

3. The power braking system, as recited in claim 2, wherein said valvemeans in the second operational mode permits air at atmospheric pressureto pass through said second inlet port into the rear chamber to createan operational pressure differential which causes the wall means to moveand pressurize the hydraulic fluid, said pressurized fluid releasingsaid spring brakes from the applied position.

1. In a braking system for a vehicle having a hydraulically operatedspring parking brake, manually controllable pumping means forcontinually supplying fluid at a constant pressure to release saidspring parking brake in a first mode of operation and for eliminatingsaid constant pressure to activate said spring brakes in a second modeof operation, said manually controllable pumping means comprising: afirst housing having an internal cavity therein; wall means including aflexible portion for dividing said cavity into a front chamber and arear chamber, said front chamber being connected to a first source offluid under pressure, said rear chamber being connected to a secondsource of fluid under pressure higher than said first pressure, saidflexible portion being adapted to move in response to expansion of saidfluid with changes in temperature in said first mode to maintain theconstant hydraulic fluid pressure on the spring brake within apredetermined range; a cylinder having an internal bore with an inletport and an outlet port, said inlet port being connected to a reservoircontaining hydraulic fluid, said outlet port being connected by aconduit to said spring brake means; piston means connected to said wallmeans and extending into the internal bore of said cylinder; poppetmeans attached to said piston means for preventing communication betweensaid reservoir and said bore during said first mode of operation and forallowing cOmmunication therebetween during said second mode ofoperation; resilient means located in said front chamber for urging saidwall means toward said rear chamber during said second mode ofoperation; a second housing having a first chamber connected to saidfront chamber, a second chamber connected to said rear chamber and acontrol chamber interconnected to said first and second chambers; valvemeans located in said control chamber having a tubular shaft surroundedby a diaphragm for separating the second chamber fron the controlchamber and a tubular valve stem extending through the second housingfrom the first chamber into the control chamber, said tubular housinghaving a resiliently positioned closure member on the end thereofadjacent said tubular shaft; sensing means located in said secondhousing and connected to said second chamber for visually indicating adifferential pressure capable of being produced across said wall meansin said first mode of operation; flow control means connected to saidsecond chamber of the second housing and the front chamber of the firsthousing for maintaining said first source of fluid under pressurecommunicated thereto at a constant pressure during both the first andsecond modes of operation; means responsive to the hydraulic fluidpressure being supplied to said spring brake means during said firstmode of operation for transmitting a first signal to an indicator deviceto inform the operator that the spring brakes are released and fortransmitting a second signal during said second mode of operation toinform the operator that the spring brakes are applied and need to bereleased before movement of the vehicle; and manual control meansconnected to said tubular valve stem for selectively engaging saidclosure member with said tubular shaft in response to an operator forsequentially communicating said first and second chambers with saidcontrol chamber to develop a corresponding pressure differential acrosssaid wall means to move said piston means in the internal bore of thecylinder and sufficiently pressurize the hydraulic fluid therein by asingle stroke to establish said first mode of operation.
 2. The powerbraking system, as recited in claim 1, wherein said first source offluid under pressure is a partial vacuum and said second source of fluidis air at atmospheric pressure, said partial vacuum being connected tosaid first inlet port and said front chamber, said air at atmosphericpressure being connected to said second inlet port, said valve means inthe first operational mode permitting partial vacuum to be communicatedfrom the first inlet port past the outlet port and into the rear chamberto eliminate the pressure differential across the wall means and releasethe pressurizing force on the hydraulic fluid to allow the spring brakesto be applied.
 3. The power braking system, as recited in claim 2,wherein said valve means in the second operational mode permits air atatmospheric pressure to pass through said second inlet port into therear chamber to create an operational pressure differential which causesthe wall means to move and pressurize the hydraulic fluid, saidpressurized fluid releasing said spring brakes from the appliedposition.